1. Field of the Invention
The present invention relates to the area of thin film analysis. In particular, the present invention relates to a method and apparatus for combining multiple thin film analysis capabilities into a single instrument.
2. Discussion of Related Art
As the dimensions of semiconductor devices continue to shrink, accurate and efficient characterization of the components forming those devices becomes more critical. Typically, the manufacturing process for modern semiconductor devices includes the formation of a number of layers or “thin films”, such as oxide, nitride, and metal layers. To ensure proper performance of the finished semiconductor devices, the thickness and composition of each film formed during the manufacturing process must be tightly controlled. In the realm of thin film analysis, three basic techniques have evolved to measure film thickness and composition.
Grazing-Incidence X-Ray Reflectometry
Grazing-incidence x-ray reflectometry (GXR), which is sometimes referred to as x-ray reflectometry (XRR), measures the interference patterns created by reflection of x-rays off a thin film. FIG. 1 shows a conventional x-ray reflectometry system 100, as described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997 to Koppel. X-ray reflectometry system 100 comprises a microfocus x-ray tube 110, an x-ray reflector 120, and a detector 130. X-ray reflectometry system 100 is configured to analyze a test sample 140 that includes a thin film layer 142 formed on a substrate 141.
Microfocus x-ray tube 110 directs a source x-ray beam 150 at x-ray reflector 120. Source x-ray beam 150 typically comprises a bundle of diverging x-rays that can have a variety of different wavelengths. X-ray reflector 120 reflects and focuses the diverging x-rays of x-ray beam 150 into a converging x-ray beam 160. Typically, x-ray reflector 120 is a singly- or doubly-curved monochromatizing crystal that ensures that only x-rays of a particular wavelength are included in converging x-ray beam 160, which is directed at thin film layer 142.
Converging x-ray beam 160 is then reflected by thin film layer 141 as an output x-ray beam 170 onto detector 130. X-ray beam 170 forms an interference pattern on the surface of detector 130 due to constructive and destructive interference of x-ray reflections at the top and bottom surfaces of thin film layer 142. Detector 130 is a position-sensitive detector that measures the varying intensity of this interference pattern. The resulting reflectivity curve of intensity versus position can then be used to calculate the thickness of thin film layer 142, as described in U.S. Pat. No. 5,619,548.
GXR is best suited for measuring thickness and electron density for films in the range of 10 A–2000 A thick. It is well matched to the barrier/seed film stacks used in BEOL (back end of line) copper interconnects. However, GXR cannot measure thicker ECP (electro-chemical plated) copper films having thicknesses greater than 1 um. Furthermore, GXR is not very good at measuring the composition of thin films—for example the composition of a barrier film such as TaN or TiSiN.
Electron Microprobe Analysis
To analyze the composition of a thin film layer, a technique known as electron microprobe (EMP) analysis is often used. EMP analysis involves the use of an electron beam (e-beam) to generate characteristic x-rays from a thin film layer. FIG. 2 shows a conventional EMP system 200 comprising an e-beam generator 210 and an x-ray detector 230. EMP system 200 is configured to analyze a test sample 240 that includes a thin film layer 242 formed on a substrate 241.
To perform an EMP analysis, e-beam generator 210 directs an e-beam 250 at thin film layer 242. The high-energy electrons in e-beam 250 cause characteristic x-rays 290 to be emitted by thin film layer 242. The properties of characteristic x-rays 290 are then measured by x-ray detector 230 to determine the composition of thin film layer 242.
Generally, x-ray detector 230 comprises either an energy-dispersive x-ray spectrometer (EDX or EDS) or a wavelength-dispersive x-ray spectrometer (WDX or WDS). In an EDX detector, the energies of the characteristic x-rays are used to determine the composition of the thin film.
FIG. 4a shows a conventional EDX detector 230a that includes a detector crystal 231 and a pulse analyzer 232. Each of characteristic x-rays 290 incident on detector crystal 231 deposits an amount of charge proportional to the energy of that particular x-ray. These charge pulses are then measured by pulse analyzer 232. Because different elements generate x-rays having different energies, the charge pulse magnitudes read by pulse analyzer 232 can be used to determine the intensity of the characteristic x-rays, which in turn can be used to determine thin film composition and thickness.
While an EDX detector provides a relatively simple means for determining the composition of a thin film layer, x-rays having closely spaced wavelengths (i.e., energies) can be difficult to distinguish. For example, an ECP copper film may be formed over a tantalum nitride barrier film. The characteristic copper x-rays (Cu—K, indicating x-rays resulting from the ionization of the K shells of the copper atoms) and the characteristic tantalum x-rays (Ta—L, indicating x-rays resulting from the ionization of the L shells of the tantalum atoms) are only separated by 100 eV, and therefore cannot be resolved by an EDX detector, which typically has a resolution limit of greater than 150 eV. Furthermore, an EDX detector cannot detect low energy x-rays, such as those emitted by the nitrogen (N—K x-rays; i.e., x-rays resulting from the ionization of the K shells of the nitrogen atoms) in a barrier film.
In contrast, WDX detectors have a much lower resolution limit of roughly 10–20 eV, and can therefore provide much more accurate measurements than an EDX detector. The low resolution limit of a WDX detector would enable Cu—K and Ta—L x-rays to be distinguished, and also enables the detection of low-energy N—K x-rays. In a WDX detector, x-rays having specific wavelengths are detected to improve the resolution of the measurement process.
FIG. 4b shows a conventional WDX detector 230b that includes an x-ray reflector 238 and a proportional counter 239. Incoming characteristic x-rays 290 are incident on x-ray reflector 238. X-ray reflector 238 is a monochromator, and disperses the incoming characteristic x-rays 290 according to Bragg's Law. X-ray reflector 238 is configured such that only those characteristic x-rays 290 having a specific wavelength are directed onto proportional counter 239. The specific wavelength is selected to be the characteristic wavelength of x-rays emitted by a particular element. Therefore, the output of proportional counter 239 can then be correlated to the concentration of the particular element in the thin film layer. Often, multiple WDX detectors are used simultaneously, with each of the multiple WDX detectors being configured to respond to a different element.
Whether an EDX or WDX detector is used, EMP analysis can be performed relatively quickly due to the intense characteristic x-rays produced by the thin film in response to the e-beam. Also, by varying the energy of the e-beam, an EMP system can “depth profile” a stack of thin film layers, allowing composition measurements to be taken at various positions thoughout the film stack. However, as film thickness in the test sample increases, the electrons in the e-beam must be raised to higher and higher energies to properly penetrate the film. For example, to penetrate 1–2 um thick ECP (electro-chemical plated) copper films, electrons with at least 50 keV energy must be used. Such high-energy electrons are difficult to produce and can damage the test sample. In addition, higher power e-beam generators increase the cost of an EMP system while decreasing overall system reliability. This is in addition to the inherent complexity introduced by vacuum environment required to generate the e-beam.
X-Ray Fluorescence
Therefore, for analysis of “thicker” thin films, a technique known as x-ray fluorescence (XRF) is often used. In place of the e-beam used in EMP analysis, XRF analysis uses a source x-ray beam to cause emission of characteristic x-rays from a thin film. The source x-rays can penetrate the film(s) in the test sample much more easily than the electrons used in EMP analysis. For example, the molybdenum x-rays (Mo—K) commonly used in XRF systems can penetrate as much as 20 um of copper, and are therefore much more efficient than an e-beam at measuring thick copper films. FIG. 3 shows a conventional XRF system 300 that includes a microfocus x-ray tube 310, an x-ray reflector 320, and a detector 330. X-ray fluorescence system 300 is configured to analyze a test sample 340 that includes a thin film layer 342 formed on a substrate 341.
Microfocus x-ray tube 310 directs a bundle of diverging x-rays 350 at x-ray reflector 320. X-ray reflector 320 reflects and focuses the diverging x-rays of x-ray beam 350 into a converging x-ray beam 360, directed at thin film layer 342. The x-rays of x-ray beam 360 cause characteristic x-rays 390 to be emitted by thin film layer 342. The properties of characteristic x-rays 390 are then measured by x-ray detector 330 to determine the composition of thin film layer 342, in a manner substantially similar to that used with respect to EMP system 200 shown in FIG. 2. Detector 330 can comprise either an EDX or WDX detector, as described previously with respect to FIGS. 4a and 4b, respectively.
Because x-rays can penetrate a material much more easily than electrons can penetrate the same material, XRF systems are generally better suited to analyze thicker films than are EMP systems. Also, a vacuum chamber is not required for the generation of the source x-rays, which simplifies the design and operation of an XRF system. However, because the source x-rays are not absorbed by the film material as well as electrons would be, and because the source x-ray beam is not as intense as an electron beam can be, the resulting characteristic x-rays in an XRF system are weaker than the characteristic x-rays in an EMP system, making measurements on those characteristic x-rays significantly slower. Also, test samples having multiple thin film layers can be problematic since the source x-rays cannot be readily “tuned” to penetrate to a specific depth.
Thus, it is clear that no single one of the aforementioned analysis techniques is ideal for all situations. However, having a different set of tools for each set of circumstances can be cumbersome and expensive. This problem can be mitigated somewhat by building multi-technique functionality into a single system. For example, Jordan Valley has produced a tool, the JVX-5000, that combines GXR and XRF capabilities. As noted previously, GXR analysis can be used to measure films less than 2000 A thick, while XRF analysis is better suited for thicker films (such as ECP copper layers). However, the Jordan Valley tool incorporates an EDX detector to measure the characteristic x-rays generated during the XRF process, thereby significantly restricting the capabilities of the Jordan Valley tool. As described previously with respect to FIG. 4a, the low resolution of an EDX detector limits its use to materials that generate x-rays having substantially different wavelengths.
Accordingly, it is desirable to provide a tool that includes multi-technique capabilities to overcome limitations associated with individual analysis techniques, while reducing instrument cost, part-count, and increasing analytical efficiency.